Pillay condemned what she called the reported “cold-blooded executions of hundreds of Iraqi hors de combat soldiers, as well as civilians including religious leaders and people associated with the government” in recent days by forces allied with the ISIL.

“Based on corroborated reports from a number of sources, it appears that hundreds of non-combatant men were summarily executed over the past five days, including surrendered or captured soldiers, military conscripts, police and others associated with the government,” Pillay said.

She said the exact number of deaths can’t yet be verified but “this apparently systematic series of cold-blooded executions, mostly conducted in various locations in the Tikrit area, almost certainly amounts to war crimes.”

Earlier in the day, U.N. Secretary-General Ban Ki-moon called reports about the slayings “deeply disturbing” and said those responsible must be brought to justice. He warned against sectarian rhetoric in Iraq that could inflame the conflict and the entire region.

The U.N. chief said he welcomed the statement on the need for unity in Iraq made by Grand Ayatollah Sayed Ali Al-Sistani, who he said “represents a deeply influential voice of wisdom and reason.”

The Islamic militants who overran cities and towns in Iraq last week have posted graphic photos on a militant website that appear to show masked fighters of the Islamic State of Iraq and the Levant, or ISIL, loading the captives onto flatbed trucks before forcing them to lie face-down in a shallow ditch with their arms tied behind their backs. The final images show the bodies of the captives soaked in blood after being shot at several locations.

Iraq’s chief military spokesman, Lt. Gen. Qassim al-Moussawi, confirmed the photos’ authenticity Sunday and said he was aware of cases of mass murder of captured Iraqi soldiers in areas held by the ISIL.

“Reports of mass summary executions by ISIL are deeply disturbing and underscore the urgency of bringing the perpetrators of such crimes to justice,” Ban said.

He called on all Iraqi leaders to ensure that their followers avoid acts of reprisal.

Ban urged the international community to unite in showing solidarity with Iraq as it confronts “this serious security challenge” and urged all to respect international humanitarian and human rights law as they try to counter terrorism and violence in Iraq.

Pillay said, according to information received by U.N. human rights employees on the ground, forces affiliated with the ISIL also executed the Imam of the Grand Mosque in Mosul on Thursday for refusing to pledge allegiance to ISIL.

Reports of additional executions of religious leaders have also been received, she said, including Saturday when 12 local Imams were allegedly executed in front of Al Israa mosque, also in Mosul, for the same reason.

“The provocative language used by ISIL, which has been talking about ‘liquidating herds of sheep’ and inciting sectarian tensions, is clearly intended to sow further chaos and bloodshed,” Pillay said. “This new wave of fighting and extreme violence is very dangerous, not just to Iraq but to the entire region.”